Maybe it's a silly question, but... are you sure the CRT is in working
order? Even if the glass isn't broken, it's possible that a blow breaks
the heating resistor. You can check this by powering on the Mac with the
case opened and looking at the end of the neck, near the circuit board.
It should light with a orangeish glow. If you don't see the glow, it may
mean that the resistor is broken, and thus, the tube's anode won't heat
to the temperature needed for it to emit electrons and no picture will
be shown in the screen, even if everything else works perfectly.
Greetings,
Antonio Rodríguez (Grijan), catching up with lists (again)
<ftp://grijan.cjb.net:21000/>
[EMAIL PROTECTED] escribió:
Well, I tested the 128k's T1 Flyback transformer in another Mac and worked
perfectly. SO that's good. But installed in the 128k it does not seem to get
power at all. That leads me to suspect the next prior component in the
circuit, the Q3 (or HOT). I tested a known good Q3 transistor in the 128k
and still no power to the T1 flyback. Using Prof. Lee's schematic, tracing
the circuit backward I see the next component is T2. Is it possible this
isolation transformer failed somehow with no visible signs? Feeding the T2
is a +12F? signal via resistor R7 and a +5V signal via a small driver
transistor Q6 that is fed via a resistor R26. The +12F seems to coming from
L1 toroid. I don't know the source of the +5V. I'm in no man's land here as
I do not know what to inspect next that is most likely responsible for power
failing to reach the T1 flyback transformer. Remember. everything else works
... the computer starts up, loads floppies, seems to be sending power to the
CRT connector, and all the voltages test correctly off the logic board. So
it would seem the power supply is working perfectly. Some obscure component
is broken somewhere on the way from the power circuit to the horizontal
circuit. HELP!
And some listers wonder why I want to see if I can use the logic board in
another Mac!
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